In many commercial and chemical processes, such as photographic film processing, metals are dissolved in fluids. The metal-laden fluids are often simply thrown away, wasting valuable metals and polluting the environment. As a result, methods and apparatus have been developed to recover these metals from the fluids in which they are dissolved.
A typical recovery apparatus includes an electrolytic cell through which metal-laden fluid is cycled. The cell typically includes two electrodes between which the fluid passes, one or more of the electrodes being rotated to provide agitation of the fluid. The fluid is stored in a batch tank and is sent through the cell by a fluid supply circuit. As fluid passes through the cell, metal plates onto one of the electrodes by virtue of an electrolytic reaction. In the case of silver, it plates onto the cathode. The metal-laden fluid is circulated through the supply circuit until the electrolytic reaction reaches equilibrium, at which point the fluid is drained from the cell and the metal is removed from the electrode.
Prior art recovery devices typically have a low flow rate within the cell as a result of the use of a single fluid circuit (the supply circuit). The low flow rate results in limited exposure of the fluid to the electrode onto which the dissolved metal plates. A batch of fluid, therefore, takes a long time to reach equilibrium.
Prior art devices do not take full advantage of the relationship between the current efficiency in a cell and the rate at which metal plates onto the electrode. These devices have lower than optimum current efficiencies, reducing the amount of metal that can be drawn out of solution in a given amount of time. This also increases the amount of time a batch of fluid takes to reach equilibrium.